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.Dd March 18, 2022
.Dt GPTBOOT 8
.Os
.Sh NAME
.Nm gptboot
.Nd GPT bootcode for UFS on BIOS-based computers
.Sh DESCRIPTION
.Nm
is used on BIOS-based computers to boot from a UFS partition on a
GPT-partitioned disk.
.Nm
is installed in a
.Cm freebsd-boot
partition with
.Xr gpart 8 .
For UEFI,
.Xr gptboot.efi 8
is used instead.
While conceptually similar, the details differ.
.Pp
When it starts,
.Nm
first reads the GPT and determines which drive and partition to
boot from, as described under
.Sx BOOTING ,
below.
If it does not find an eligible partition, or if the user hits a
key within three seconds,
.Nm
switches from auto-boot to interactive mode.
Interactive mode allows manual selection of the disk, partition,
filename, and boot option flags, as described in
.Xr boot 8 .
.Sh IMPLEMENTATION NOTES
The GPT standard allows a variable number of partitions, but
.Nm
only boots from tables with 128 partitions or less.
.Sh PARTITION ATTRIBUTES
.Nm
checks and manages several attributes of GPT UFS partitions.
.Bl -tag -width ".Cm bootfailed"
.It Cm bootme
Attempt to boot from this partition.
If more than one partition has the
.Cm bootme
attribute set,
.Nm
will attempt to boot each one until successful.
.It Cm bootonce
Attempt to boot from this partition only one time.
Setting this attribute with
.Xr gpart 8
automatically also sets the
.Cm bootme
attribute.
Multiple partitions may have the
.Cm bootonce
and
.Cm bootme
attributes set.
.It Cm bootfailed
The
.Cm bootfailed
attribute marks partitions that had the
.Cm bootonce
attribute set, but failed to boot.
This attribute is managed by the system.
See
.Sx "BOOTING"
and
.Sx "POST-BOOT ACTIONS"
below for details.
.El
.Sh USAGE
For normal usage, the user does not have to set or manage any of the
partition attributes.
.Nm
will boot from the first UFS partition found.
.Pp
The
.Cm bootonce
attribute can be used for testing an upgraded operating system on
an already-working computer.
The existing system partition is left untouched, and the new version
of the operating system to be tested is installed on another partition.
The
.Cm bootonce
attribute is set on that new test partition.
The next boot is attempted from the test partition.
Success or failure will be shown in the system log files.
After a successful boot of the test partition, a user script can check
the logs and change the
.Cm bootme
attributes so the test partition becomes the new system partition.
Because the
.Cm bootonce
attribute is cleared after an attempted boot, a failed boot will not
leave the system attempting to boot from a partition that will never
succeed.
Instead, the system will boot from the older, known-working operating
system that has not been modified.
If the
.Cm bootme
attribute is set on any partitions, booting will be attempted from them
first.
If no partitions with
.Cm bootme
attributes are found, booting will be attempted from the first UFS
partition found.
.Sh BOOTING
.Nm
first reads the partition table.
All
.Cm freebsd-ufs
partitions with only the
.Cm bootonce
attribute set, indicating a failed boot, are set to
.Cm bootfailed .
.Nm
then scans through all of the
.Cm freebsd-ufs
partitions.
Boot behavior depends on the combination of
.Cm bootme
and
.Cm bootonce
attributes set on those partitions.
.Bl -tag -width ".Cm bootonce + .Cm bootme"
.It Cm bootonce + Cm bootme
Highest priority: booting is attempted from each of the
.Cm freebsd-ufs
partitions with both of these attributes.
On each partition, the
.Cm bootme
attribute is removed and the boot attempted.
.It Cm bootme
Middle priority: booting is attempted from each of the
.Cm freebsd-ufs
partitions with the
.Cm bootme
attribute.
.El
.Pp
If neither
.Cm bootonce
nor
.Cm bootme
attributes are found on any partitions, booting is attempted from the
first
.Cm freebsd-ufs
partition on the disk.
.Sh POST-BOOT ACTIONS
The startup script
.Pa /etc/rc.d/gptboot
checks the attributes of
.Cm freebsd-ufs
partitions on all GPT disks.
Partitions with the
.Cm bootfailed
attribute generate a
.Dq boot from X failed
system log message.
Partitions with only the
.Cm bootonce
attribute, indicating a partition that successfully booted, generate a
.Dq boot from X succeeded
system log message.
The
.Cm bootfailed
attributes are cleared from all the partitions.
The
.Cm bootonce
attribute is cleared from the partition that successfully booted.
There is normally only one of these.
.Sh FILES
.Bl -tag -width /boot/gptboot -compact
.It Pa /boot/gptboot
bootcode binary
.It Pa /boot.config
parameters for the boot blocks
.Pq optional
.El
.Sh EXAMPLES
.Nm
is installed in a
.Cm freebsd-boot
partition, usually the first partition on the disk.
A
.Dq protective MBR
.Po
see
.Xr gpart 8
.Pc
is typically installed in combination with
.Nm .
.Pp
Install
.Nm
on the
.Pa ada0
drive:
.Bd -literal -offset indent
gpart bootcode -b /boot/pmbr -p /boot/gptboot -i 1 ada0
.Ed
.Pp
.Nm
can also be installed without the PMBR:
.Bd -literal -offset indent
gpart bootcode -p /boot/gptboot -i 1 ada0
.Ed
.Pp
Set the
.Cm bootme
attribute for partition 2:
.Bd -literal -offset indent
gpart set -a bootme -i 2 ada0
.Ed
.Pp
Set the
.Cm bootonce
attribute for partition 2, automatically also setting the
.Cm bootme
attribute:
.Bd -literal -offset indent
gpart set -a bootonce -i 2 ada0
.Ed
.Sh SEE ALSO
.Xr boot.config 5 ,
.Xr rc.conf 5 ,
.Xr boot 8 ,
.Xr gpart 8
.Sh HISTORY
.Nm
appeared in
.Fx 7.1 .
.Sh AUTHORS
This manual page was written by
.An Warren Block Aq wblock@FreeBSD.org .
